This invention relates generally to frames for bicycles. More particularly, the invention is directed to a mountain bicycle frame.
Conventional bicycle frames use tubes joined into closed frameworks to interconnect the components (front fork/handlebar assembly, seat, crankset, rear wheel) and transmit the loads there between. The tubes also keep the components in spacial relation, i.e., they maintain the components relative proximity.
The traditional bicycle frame includes a top tube, a relatively short head tube, a seat tube, a down tube, a bottom bracket, a pair of chain stays, a pair of dropouts, and a pair of seat stays. The top tube connects at its front end to the top of the head tube, and extends backward to the top of the seat tube. The down tube connects at the front end to the bottom of the head tube and extends downwardly and rearwardly to the bottom bracket where the seat tube and the down tube are connected. The pair of chain stays extend rearwardly from the bottom of the seat tube to the dropouts. The pair of seat stays connect directly or indirectly to the top of the seat tube. The seat stays extend backward from the seat tube to the dropouts. The seat stays generally include two seat stay tubes that converge from the dropouts toward the seat tube. The dropouts support the rear wheel axle. This conventional frame design is called the xe2x80x9cdiamondxe2x80x9d frame, because when viewed from the side, the top tube, down tube, chain stays, and seat stays enclose a diamond-shaped space.
Such frames are used for both road bikes and mountain bikes. The terrain that a bike is used on dictates the load requirements for the bike and the performance characteristics of the components. For example, since mountain bikes are used on rough terrain and often are ridden over roots and jumps they must be more durable than road bikes, which encounter relatively smooth surfaces.
With respect to performance characteristics, due to the riding conditions encountered on mountain bikes, mountain bike brakes must provide more braking force than road bike brakes. To that end, mountain bikes are typically equipped with cantilever brakes. In order to mount cantilever brakes for use on a rear wheel, the seat stay tubes must include brake bosses. The brake bosses extend generally perpendicular to the seat stay tubes. Cantilever brakes include two separate brake arms that are pivotally mounted on the brake bosses. A cable links the arms to a brake lever mounted on the handlebars. The other end of each arm has a pad mounted thereto. When the lever is depressed, the cable causes the upper end of each arm lo move outward Consequently, the lower end of each arm and the pads connected thereto move inward and contact opposite sides of the rim of the rear wheel. Cantilever brakes apply a large braking force to the rim.
More over, cantilever brakes exert large lateral forces on the seat stays, which has a tendency to separate the seat stays from one another. In order for the seat stays to endure these forces and to minimize the tendency for the seat stays to spread, they must meet certain stiffness requirements. The stiffness requirements have been met bending the seat stay tubes into various geometries, providing large diameter seat stay tubes, providing tubes with increased wall thicknesses and/or using materials that exhibit increased stiffness. These techniques can be used separately or in different combinations. However these techniques can add weight to the frame, which is undesirable.
Some mountain bikes today are equipped for use with cantilever brakes or disc brakes. The seat stays for such frames are robust enough to with stand use with cantilever brakes, and the seat stays have bosses for receiving cantilever brakes. Thus, the techniques mentioned above are used to ensure that the seat stays are stiff enough for use with the cantilever brakes.
Road bikes, on the other hand, typically use caliper brakes. In order to mount caliper brakes to a frame, the seat stays generally include a seat stay bridge that extends between each of the seat stay tubes. The bridge has a bore defined therein for pivotally mounting the caliper brakes. The caliper brakes apply less braking force than cantilever or disc brakes and require less robust seat stays. Thus, the seat stays for road bikes are usually less stiff and lighter than mountain bikes.
The need exists for a lighter weight mountain bike frame that meets the performance characteristics for mountain bikes.
According to the present invention a mountain bike frame for use with a rear wheel is disclosed. The frame includes a head tube, a top tube, a down tube, a bottom bracket, a seat tube, a pair of dropouts, a pair of chain stay tubes, and a pair of seat stay tubes.
The top tube extends rearwardly from the head tube. The down tube extends substantially rearwardly and downwardly from the head tube. The seat tube has a first portion connected to the top tube and a second portion coupled to the down tube, such that the head tube, top tube, down tube and seat tube form a first substantially triangular first region. The bottom bracket is connected to the down tube and seat tube. Each chain stay tube in the pair is spaced from the other and extends generally rearwardly from the bottom bracket to the associated dropout. Each seat stay tube in the pair is spaced from the other and coupled to the top tube so that the seat stay tubes extend generally rearwardly and downwardly from the top tube to the associated dropout. The seat stay tubes define a gap distance between the inner surface of the seat stay tubes at a tire intersection point that at a minimum is greater than about 2 inches. The seat stay tubes also have a modulus of elasticity value and a moment of inertia value such that the modulus of elasticity value multiplied times the moment of inertia value is between about 1.0 lbxc3x97inches2 and about 10 lbxc3x97inches2.
In one embodiment, the frame is for use with a rear disc brake coupled to one of the seat stays.
In yet another embodiment, the present invention is directed to a bicycle including the above frame.